Fix incorrect use of a singleton list (#3914)

## What type of PR is this? (check all applicable)

- [x] Refactor
- [ ] Feature
- [ ] Bug Fix
- [ ] Optimization
- [ ] Documentation Update
- [ ] Community Node Submission
      
## Description

`search_for_models` is explicitly typed as taking a singular `Path` but
was given a list because some later function in the stack expects a
list. Fixed that to be compatible with the paths. This is the only use
of that function.

The `list()` call is unrelated but removes a type warning since it's
supposed to return a list, not a set. I can revert it if requested.

This was found through pylance type errors. Go types!

invokeai/app/api/routers/models.py

@@ -298,7 +298,7 @@ async def search_for_models(
 )->List[pathlib.Path]:
     if not search_path.is_dir():
         raise HTTPException(status_code=404, detail=f"The search path '{search_path}' does not exist or is not directory")
-    return ApiDependencies.invoker.services.model_manager.search_for_models([search_path])
+    return ApiDependencies.invoker.services.model_manager.search_for_models(search_path)
 
 @models_router.get(
     "/ckpt_confs",

invokeai/app/services/model_manager_service.py

@@ -600,7 +600,7 @@ class ModelManagerService(ModelManagerServiceBase):
         """
         Return list of all models found in the designated directory.
         """
-        search = FindModels(directory,self.logger)
+        search = FindModels([directory], self.logger)
         return search.list_models()
 
     def sync_to_config(self):

invokeai/backend/model_management/model_search.py

@@ -98,6 +98,6 @@ class FindModels(ModelSearch):
 
     def list_models(self) -> List[Path]:
         self.search()
-        return self.models_found
+        return list(self.models_found)